1. Field of the Invention
This invention lies in the field of composites of silicon nitride and silicon carbide, and relates in particular to methods for preparing these composites as materials of high relative density, high strength, and high creep resistance.
2. Description of the Prior Art
Composites of silicon nitride and silicon carbide have been widely investigated as materials for load-carrying structural components that require strength and toughness and that can withstand high temperatures. The high strength and toughness of these composites, particularly at high temperatures, as well as their high oxidation resistance, have been attributed to the silicon carbide component, while the silicon nitride component limits the thermal expansion of the composite to a low value. These characteristics are highly desirable for a wide variety of applications extending from microelectromechanical devices (MEMS) to materials of construction for high-stress equipment such as heat engines, cutting tools, wear and friction surfaces, and space vehicles. These qualities are attributable to several factors, prominent among which are the density and microstructure of the composite. The strength of the composite, for example, increases with the relative density.
A property of these composites that has been difficult to control in the prior art is creep resistance. Reports of the effect of the inclusion of silicon carbide on the creep resistance have indicated inconsistent results, and it is believed that the inability to control creep resistance has been due at least in part to efforts for achieving high relative density.
Prior art methods for preparing dense silicon nitride/silicon carbide composites typically involve the consolidation and densification of powder mixtures of silicon nitride and silicon carbide. The typical consolidation and densification methods are hot-pressing, gas-pressure sintering, and hot isostatic pressing. The composites produced by these methods generally have microstructures consisting of micron-sized or sub-micron-sized grains of both silicon nitride and silicon carbide crystals with inclusions of nano-sized crystals of silicon nitride dispersed through the micron-sized or sub-micron-sized grains. The term “micron-sized” refers to grains having diameters that are greater than 1 micron, “sub-micron-sized” refers to grains having diameters within the range of 100 nm to 1,000 nm, preferably 150 nm or above, and “nano-sized” refers to grains whose diameters are less than 100 nm, particularly 50 nm or below. To increase the degree of densification that occurs during these procedures, densification aids have been used, notably metal oxides that are liquid at the sintering temperature. Examples of such metal oxides are magnesium oxide (MgO), alumina (Al2O3), yttria (Y2O3), lithium oxide (LiO2), and rare earth oxides such as ceria (CeO2). Alumina, yttria, or a combination of alumina and yttria are most often used.
While the metal oxides improve the density of the product, they tend to interact with the silicon oxide (SiO2) films that are often present on the surfaces of the powder particles. These films are typically formed as a residue of the nitridation reaction by which the silicon nitride powder is produced from silicon starting material. Once formed, the silicon oxide films interact with an alumina or yttria densification aid to produce an oxy-nitride glass at the interfaces between the silicon nitride and silicon carbide particles. The glass melts at a temperature lower than the melting temperature of either silicon nitride or silicon carbide. This lowers the creep resistance of the final, densified product. Starting powder mixtures that are prepared by methods other than the nitridation of silicon also suffer from a measurable creep rate when a metal oxide is used for densification, since the metal oxide itself demonstrates glassy phase behavior.
The present invention seeks to address these problems by providing a method for producing a highly dense silicon nitride/silicon carbide composite with a creep rate that is either very low or below the measurement limits of current creep testing equipment.